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Final Report 3-47 September 2000 Allred (1997) studied channel narrowing and vertical accretion in the Green River at the Green River, Utah, gage and described the process by which in-channel deposits become stabilized. The stabilization process included the following steps: (1) emplacement and accretion of a lateral bar as large amounts of sediment are moved through the system, (2) low flood magnitude in years following bar emplacement, (3) rapid encroachment of riparian vegetation onto the exposed bar surface, (4) stabilization of the bar through extensive root system development, and (5) continued vertical accretion of the bar surface during periods of inundation when existing vegetation captures additional sediment. Channel narrowing at this location occurred from 1930 to 1938; rapid accretion occurred from 1957 to 1962; and further narrowing occurred after 1962 (Allred 1997; Allred and Schmidt 1999). The 2-year flood decreased from 1,190 m3/s for the period of 1895 to 1929, to 800 m3/s between 1930 and 1957, and finally to 635 m3/s after dam closure. This research indicates that channel narrowing occurred in response to weather changes and as vegetation (primarily tamarisk Tamarix ramosissima) invaded and stabilized newly formed inset floodplain deposits. The large floods of 1983 and 1984 did not reverse the narrowing trend at this site but instead resulted in the deposition of sediments at higher elevations. O'Brien (1998) proposed in-channel maintenance flows for the Ouray portion of Reach 2 and the Canyonlands portion of Reach 3 on the basis of calculated incipient motion values for sand particles. These proposed in-channel maintenance flows would perform the physical process of reworking large sand bars and returning the sand to the deeper portions of the channel bed after the spring peak flow, thus preventing vegetation encroachment, bar attachment, and channel narrowing. Calculated in-channel maintenance flows ranged from 142 to 467 m3/s (mean 235 m3/s) in Reach 2 and 170 to 261 m3/s (mean 208 m3/s) in Reach 3 (O'Brien 1998). These moderate in-channel flows would assist in keeping the channel active, reworking in-channel sand bars, and reducing the impact of sediment deposition in sensitive habitat areas. 3.6.5 Floodplain Inundation Floodplains develop along rivers where the valley floor is extensively covered with alluvium. The normal-flow channel, carved in the alluvium, is flanked by this low-relief surface that becomes part of the river bed during high-flow periods. Floodplains are primarily depositional landforms formed by lateral and vertical accretion of sediment deposits. These areas serve as important nursery and growth and conditioning habitats for endangered fishes in the Green River, particularly the razorback sucker. The frequency and extent of floodplain inundation vary considerably along the Green River and are largely a function of site-specific channel morphology (including the presence or absence of natural or manmade levees). Irving and Burdick (1995) conducted an inventory, largely on the basis of aerial photography, of potential flooded bottomland habitats in the Green River. They determined that approximately 644, 3,500, and 3,300 ha were present in Reaches 1, 2, and 3, respectively. In